Earth Notes: Installing Solar PV for Zero-Carbon Electricity @Home (2009)

Updated 2022-09-19.
Does solar PV really work? Does it also make you rich, sexy and smug? #microgen #solar #PV

Summary

If you want to do your bit to tackle climate change then you should know that you can get more bang for your buck with things other than solar PV, such as energy conservation and avoiding flying.

If you do solar PV carefully then you can probably recoup your money after ten years, and if you want to generate significant amounts of energy to run household appliances, and you already have a reliable domestic electricity supply, then a simple "grid-tie" system is likely to to be the most cost-effective.

What Should be Considered Before/Instead?

Here's the disclaimer first... Assuming that you're considering solar panels to help fight climate change rather than simply impress the neighbours with some rooftop "bling", then though solar PV is a good thing you should consider the following which are easier and give more bang for your buck even if not as ostentatious:

  1. Save electricity by turning off unused lights and appliances and when replacing get the most energy-efficient versions that you can. £1 spent conserving may achieve as much as £10 spent on PV, and you might even be able to generate more than you use (we do).
  2. Improve your home's insulation by draught-proofing, improving your heating controls, turning down the thermostat and wearing a sweater, etc. All of these are really part of the previous 'conserve energy', so even if you don't heat with electricity you'll still likely save more carbon pollution/emissions than solar panels can.
  3. Solar hot water (also called "solar thermal" or "solar DHW" or "SDHW") is often considerably cheaper than PV, simpler and very effective.
  4. Drive and fly less, and eat less meat to reduce your carbon footprint more than PV probably can.

Having said that, once a standard 'grid-tied' solar system is installed it is more-or-less zero-maintenance, zero-noise, zero-fuss, and under the UK government's 2010 'Feed-In-Tariff' may well make you money (and help protect yourself against fuel price rises) as well as playing a significant part in reducing climate change. And I'm rather proud of our solar PV installation!

A small caveat for solar thermal is that energy that you can't use (do you really want a hot bath on a hot summer's day?) is wasted, whereas excess energy from solar PV goes into the grid and helps reduce generation to cover your neighbours' consumption too. I especially like the fact that when on holiday in the UK the electricity that I generate at home 'follows me about' via the National Grid for charging my camera or my phone or whatever.

And of course you can deploy more than one option such as improved insulation, thermostatic radiator valves, efficient appliances, saving electricity, and solar PV, which is what we've done...

What Are The Solar Electricity Options?

The three main options are:

and I'll explain each the pros and cons of each. We have both off-grid and grid-tied systems at home.

Off-Grid

An off-grid system is one with no connection to the 'grid' ie mains electricity.

"Off-grid" encompasses everything from a solar-powered pocket calculator to an entire remote house with battery backup and no external 'mains' supply.

Often but not always a rechargeable battery may feature in such a system. For a device such as the calculator there may well be no backup battery so you can only use it in the light, but since you can only read its display in the light, that is not a problem. Such battery-less systems can be cheaper and lighter and smaller, but may be less convenient to use. The remote house would almost certainly benefit from battery storage to run lights or TV or other appliances at night. Such a system may have multiple forms of generation for reliability, such as a wind turbine or a stream-driven turbine or diesel backup.

Simple off-grid systems can be assembled piecemeal and relatively inexpensively and with parts from retail stores and Internet suppliers. For example, I put together in mid-2007 a small off-grid solar PV system starting with a small (20Wp, ie 20 Watt peak) solar panel, solar regulator, and 40Ah 12V sealed (no-maintenance) battery, with the aim of using that to power efficient lighting in my home office thus reducing my use of the mains accordingly. This has been largely successful (I am working by an LED lamp from it now), and I have extended it with panels from various sources to about 200Wp and a newer and smarter solar regulator, and additionally it now powers my main Internet servers, though I had to work hard to reduce their consumption 100-fold to get there!

So, in this case the solar PV is not connected to the grid at all (and like solar thermal, any 'excess' unused energy is wasted) but manages to displace consumption from the grid.

This is a nice hobby, and with care may save more grid emissions then is 'embedded' in the manufacture of the equipment. But expanding to a size sufficient to cover that rural house with the appliances that we've come to expect with grid living is hard due to several factors, even with optimal condition and plenty of space:

Now you see why grid is good...

Of course, for mobile applications, or where the grid is not available, cost may not be the main issue at all.

Smaller ground-based off-grid systems might employ a tracker so that your solar PV follows the sun and maximises your output, but note that this is something mechanical to break, especially in high winds, and will tend to have most effect in the summer (when the arc of the sun across the sky is greatest) when you are most likely to have an excess of energy anyway. My advice is just to buy more panels instead in most cases.

Grid-Tie

Grid-tie (or 'grid-tied') solar is so called because it is tied to the grid, and does not function stand-alone.

Typically a domestic grid-tie is mounted on your roof with the PV panels wired to the system 'inverter' (via a cut-off switch) and to the mains on your side of the meter (via another cut-off switch).

A grid-tie system inverter 'listens' to the mains, and when solar power is available carefully and synchronously injects that power into the mains (converting from DC to AC as it does so) where it is available to power local appliances with any unused excess 'spilling' out to the grid for others to use.

If there's a power cut the grid-tie system shuts itself down for safety, for example to avoid hurting any engineers trying to repair the fault and who might otherwise assume the cable to your house to be dead.

A grid-tie inverter can be small and have high efficiency (over 90%) and the absence of batteries saves cost and energy and space, and is a relatively good way of displacing generation and thus emissions.

(Grid-tie inverters exist for other renewable sources such as wind and hydro.)

We have a grid-tie solar system installed in in two phases, and using the most efficient (not cheapest) available solar panels to generate as much as possible on our small roof. (As I discuss below, if you have lots of space, you can minimise cost instead.)

These days for most purposes you can install solar microgeneration without planning permission (using "permitted development rights"), though we still had to abide by covenants in our deeds and ask permission. That assumes panels/tubes fixed parallel to the roof (no tracking for example).

Then, because we had the design and installation done by Evo Energy which has "Part-P" certification, no building inspection (etc) was required. This simplifies the process and keeps insurers happy, but you do have to trust the installer not to wreck your roof and to check that it is strong enough to cope with (for example) large winds trying to turn your new panels into sails...

(As ever, prudence suggests having at least a couple of firms quote having come and inspect your house in person, for money/viability and safety reasons.)

Any fixed solar system will generally generate most energy over a year on a south-facing at a ~30° pitch in the UK, but our east- and west- facing roof surfaces at 24° only cost about 20% performance at worst and generate what I think is a flatter more grid-friendly output in summer.

Avoid shading from trees and nearby buildings; even shading a very small part of a panel or set of panels can drastically reduce output (by 90% or more). Having said that, I installed our second round of PV knowing that it would be significantly shaded by a large tree to the north-east in Autumn; in the summer the sun is too high in the sky for the tree to be in the way and in the winter the loss of leaves reduces the problem somewhat.

The upshot is that a obstructed or non-south-facing roof to put PV on will increase costs for a given output, but is not necessarily fatal.

Grid-tie with Battery Backup

It is possible to combine the virtues (and costs) of grid-tie and off-grid with 'auto islanding' grid-tie inverters with battery backup.

When the grid up they work just like grid-tie and keep the batteries charged. Energy can flow to or from the grid as usual depending on how much is being generated compared to how much is being used, and the batteries will be kept charged from local solar or the grid.

If the grid fails, however, ie there is a power cut, then the inverter disconnects the house from the grid (thus protecting anyone working on the lines), and draws energy from the batteries and solar PV to power the house or uses excess energy to top up the batteries. If there is a continuing excess then because it can't be exported to the grid, the inverter may force other grid-tie inverters on its side to shut down.

Such a backed-up grid-tie system may be valuable in an area with frequent power-cuts for whatever reason, as it serves as a whole-house UPS (Uninterruptible Power System). The battery capacity could be relatively small compared to off-grid if it is primarily to ride out short grid outages rather than multiple mid-winter cloudy days.

However, the inverter and batteries will cost more (and take more space) than a simple grid-tie system, so with London's very reliable domestic power it was not useful for our house, and might simply be a waste of energy undermining conservation.

It is possible to retrofit such a battery backup system later, smart enough to make use of simple grid-tie microgeneration already installed.

Rules and Tools?

Do you have a few square metres (6--20) roof (or ground) with no shading, preferably south-facing else shallow-pitch east/west roof is fine? Watch for a few days, preferably in winter and summer, to check for shading from buildings, trees, pipes or aerials.

Having located a suitable well-lit area you may wish to work out how much you could generate, given its size and which way it faces and its slope/pitch. The free on-line PVGIS Solar Irradiance Data tool is very good indeed and gave me predictions within about 10% of what our system actually generates.

An installer may well use other tools, for example to help them choose the best panels and inverter for your site.

As mentioned above, reasonable-size conventional solar PV does not usually now need planning permission (except in conservation areas and the like), for example see this Green Building Forum thread, though we had covenants in our freehold meaning that we had to ask our local council for permission anyway. Happily our council is supportive of microgeneration and were positively helpful. If you use a "Part-P competent" installer there will be no further red-tape from building control or inspection.

You do need the agreement of your electricity supplier if you are to be paid for what you generate, and will need an OFGEM-approved total generation meter. You will separately need to tell the DNO (Distribution Network Operator) that runs the electricity supply to your door at least 24 hours before installation, but it basically cannot refuse if your inverter supplies less than 16A per phase or about 3.7kW. Your DNO is quite often not your supplier these days, but your installer should be able to sort out that part.

Variations

There are a few possible variants worth mentioning, even if not very common:

If you have a lot of well-lit ground space you can consider ground-mounted solar in addition to or as well as roof-mounted. Most of us are not doing anything much else with our roof and other space may be limited of course.

It is possible to combine solar PV and solar thermal in one unit, which in principle may do better than either in a given space (eg solar PV generates less when it gets too hot; the water can cool it), but such units are complex and expensive and as different people tend to put in solar thermal and PV systems, you run the risk of confusing both.

If space is tight as it was for us then you can buy the most 'efficient' PV (that is the most energy captured per unit area for a given light level) but it is likely to be relatively expensive. These high-efficiency panels tend to be crystalline silicon with fancy features. But if space is not especially an issue for you then you can minimise cost by, for example, going with cheaper thin film products that will take maybe twice the area to capture the same energy as the most efficient panels, but at significantly lower cost per unit of energy.

Tesco enters Stage Left

The major high-street retailer Tesco now has its own Tesco Renewable Energy offering for solar PV (and thermal) and insulation. This moves the provision of PV from jobbing craftsmen to big-bucks economies of scale with guarantees that are clearly likely to be enforceable.

A friend actively considering PV had the news items from Tesco's PR pointed out to him 2010-04-06, registered 2010-04-07, had a sensible response 2010-04-08, and said that someone would be sent out to see him soon, unlike many of the smaller players reluctant to make numerous speculative visits to potential customers. Let's hope at least that Tesco will set a benchmark for price and reliability.

A few days later Tesco seemed to have lost the plot somewhat, asking again for data such as roof slope and orientation already collected, and appeared to be possibly stalling for time.

After much flapping about Tesco appointed a company to handle the quotation, and my friend obtained competing quotations from other suppliers.

To cut a long story short, all the suppliers decided that the roof shape was awkward (with some difficult shading issues) though three quotes ranged from about £4.2/Wp (for a 1.4kWp Moser Baer and Sunny Boy system) allowing for scaffolding already in place, through £6.2/Wp from Tesco's appointed supplier (for a 1.75kWp Sharp and Soladin solution), to £7.8/Wp (for a 1.29kWp Sanyo HIT and Sunny Boy scheme).

As of June 2010 my friend has decided not to go ahead with any of them, in part because he was disconcerted by some poor customer service and silly basic arithmetic mistakes (eg how many panels fit on a roof), though he has fitted some support battens and lots of insulation in a flat roof that was being renovated and might be suitable for some sort of solar, PV or thermal, in future, and is considering a heat-pump to replace his oil-fired combi under the RHI. (I still think that he has potential for PV on at least one area...)

Farm, Listed Building, Tax, and Other Fun

There is a farm that I know where the owner already has solar thermal and burns wood for heat from attached woodland, and where solar PV should be a good possibility.

Because there is plenty of roof space on farm buildings, not visible from the road (ie from off the premises), thin-film is a viable option to save some capital.

Also, as the grid there is quite weak, pumping a bit of energy in at the end of the long piece of string might significantly help stability when the sun is out!

I'm helping the farmer explore possibilities as of mid-2010...

A brief chat with the DNO suggested that they would be happy with up to 5kW of microgeneration on the circuit.

I've requested quotes from Tesco and two suppliers/installers for 4kWp systems for the maximum FiT returns, mentioning thin-film as a possibility we'd like to explore.

A few questions lurk however:

Some Weeks On

A few notes:

As of mid-Sept we're about to write our pre-application enquiry letter to the conservation team of the local authority. A response should be with us in about 10 working days.

Pre-Application Enquiry Response

The response was on time but on the face of it not good; in the light of recent planning decisions, and given the character of the structure we proposed placing the panels on, the conservation officers did not support our original plans, but seemed keen to work with us to find some acceptable way to get renewables installed.

So, taking a different tack, one of the PV companies involved has run with the ball, pitching for a much larger install on the roof of a newer (not conservation material!) pole-barn, and is talking to the conservation officers itself.

Up and Running

At one stage it looked as if we'd get through on permitted development rights, but those has been explicitly given up as part of previous work, so a full planning application was put in, and granted, and a PV system was up and running in Autumn 2011.

How Much CO2 and Wonga Saved?

Working out how green your halo will become is quite complicated. Even if you produce more electricity over the course of a year than you use, generally you will still be importing electricity some from the grid, some of which will likely have taken fossil fuels to produce. In particular with a conventional simple grid-tie system, unless you turn everything off at sunset until sunrise the next day, you will be importing from the grid overnight, even in high summer...

With our grid-tie system, even as slight net generators over the year, we are big exporters on most summer days and importers in winter and at night. We seem to use on average maybe ~30% of our PV-generated electricity locally, ie without it getting near the grid, which is probably maximally efficient. The rest is exported because we don't use it at the time we're generating, even though we'll need it (eg for lighting at night) when we can't generate.

Now, it may be that overnight when demand is low much/all of each kWh imported may come from zero-carbon sources such as baseload nuclear, wind, and in future others such as tidal, biomass, etc. Also, when exporting excess during the day it reduces the need for other generation on the grid, typically more carbon-intense than average. So you could do a complicated sum noting the times and amounts of import and export and the grid intensity then, and work out a complicated carbon balance that would make a bean-counter proud. One day when feeling patient I may attempt such a thing...

However, as a reasonable approximation simply net off imported and exported electricity and multiply by average grid intensity for the year. Thus, if you export more than you import over a year then you have a negative carbon footprint you can take off something else, otherwise your total carbon footprint is still reduced by what you exported.

Note however that since winter grid carbon intensity is higher than summer, then if you are only exporting a relatively small annual excess it may be pragmatic to regard your electricity carbon footprint as zero rather than negative in return for the vastly simplified calculations.

As to the money...

If you are lucky enough to have net metering as we did by accident for a while, where every exported kWh/unit is credited back to you at full retail price, you may end up with a zero bill (very unlikely to be allowed to go negative). However, net metering is frowned on because (on top of other reasons) having meters go backwards upsets the industry's accounting systems!

More typically any money that you make will be some combination of the following (as regulations change, so will the details):

So, you'll insulate yourself from a proportion of rising energy prices, which in conjunction with behaviour changes and when you run appliances, might halve your electricity bills or much better; we are probably saving over £1000 per year compared to our previous heavy usage and will now probably pay ~£200 for 2010 even on a premium 'green' tariff (so that what we do import is as green as possible) and that's even though we now have two kids.

You may also receive payments for generation and export as given above, which with care and the above savings and grants/loans may have a solar PV installation repay itself in (say) 10 years. Given that a decent system will probably last upwards of 25 years, and that the savings it makes may be more reliable or larger than investment of the same money in turbulent financial markets, then it might be thought of as a relatively-safe government-regulated exposure to investments in energy itself. Clearly you should consult a qualified financial adviser and/or do your own sums, but I do also view my solar investment in this way, like a 'green fund', even though the climate change issues are the most important drivers for me.

Against those savings/earnings, allow for the fact that although PV panels should last 20 years or more (though with a slowly-reducing output each year) the inverters which put the energy into the mains typically wear out sooner, maybe on average every 10 years, and will thus need replacing before the panels. (You should also be prepared for some very minor manual maintenance such as dusting the heatsinks and similar every year or so to prolong inverter life.) You will also probably want to ensure that your expensive equipment is insured against catastrophe, which although was free on our house policy, may not be for you.

Free Systems

The FiT is designed to be a very decent investment, so much so that several companies (see Links) will install solar for free, giving you the savings from your bill and a green halo, financing the installation from the FiT and export fees which you sign over. If you can find the capital you should do a PV installation yourself, but if you can't then you could consider one of the deals on offer.

My primary interest at the moment is getting as much PV on roofs as possible, several GW if possible, and this is one good route to that end.

I have drafted a contract that my company might issue to put solar on someone else's roof. Though you must take legal advice of your own and I cannot accept any liability for use of this text, you are welcome to use any elements of the text that you might find helpful in constructing such a contract of your own and promoting responsible installation of solar PV.

Sexy and Smug?

Well, it seems that solar PV makes your house more desirable at least, and if you get that draught-proofing done you can be snug if maybe not smug...